Formation dry‐out from CO2 injection into saline aquifers: 1. Effects of solids precipitation and their mitigation

F ORMATION D RY -O UT FROM CO 2 I NJECTION INTO S ALINE A QUIFERS . P ART 1, E FFECTS OF S OLIDS P RECIPITATION AND THEIR M ITIGATION Karsten Pruess 1 and Nadja Muller 2 Earth Sciences Division, Lawrence Berkeley National Laboratory University of California, Berkeley, CA 94720 Shell International Exploration and Production B.V. - Exploratory Research Rijswijk, The Netherlands Abstract Injection of CO 2 into saline aquifers may cause formation dry-out and precipitation of salt near the injection well, which may reduce formation porosity, permeability, and injectivity. This paper uses numerical simulation to explore the role of different processes and parameters in the salt precipitation process, and to examine injection strategies that could mitigate the effects. The main physical mechanisms affecting the dry-out and salt precipitation process include (1) displacement of brine away from the injection well by injected CO 2 , (2) dissolution (evaporation) of brine into the flowing CO 2 stream, (3) upflow of CO 2 due to gravity effects (buoyancy), (4) backflow of brine towards the injection point due to capillary pressure gradients that oppose the pressure gradient in the CO 2 -rich (“gas”) phase, and (5) molecular diffusion of dissolved salt. The different mechanisms operate on a range of spatial scales. CO 2 injection at constant rate into a homogeneous reservoir with uniform initial conditions is simulated in 1-D radial geometry, to resolve multi-scale processes by taking advantage of the similarity property, i.e., the evolution of system conditions as a function of radial distance R and time t depends only on the similarity variable R 2 /t. Simulations in 2-D vertical cross sections are used to examine the role of gravity effects. We find that counterflow of CO 2 and brine can greatly increase aqueous phase salinity, and can promote substantial salt precipitation even in formations with low dissolved solids. Salt precipitation can accentuate effects of gravity override. We find that injecting a slug of fresh water prior to commencement of CO 2 injection can reduce salt precipitation and permeability loss near the injection well.